Method of melting and supplying glass along a feeder duct



P. A. M. GELL Sept. 3, 1968 METHOD OF MELTING AND SUPPLYING GLASS ALONG A FEEDER DUCT 2 Sheets-Sheet 1 Filed June 9, 1964 P. M. GEI.L 3,400,204

METHOD OF MELTING AND SUPPLYING GLASS ALONG A FEEDER DUCT Sept. 3, 1968 2 Sheets-Sheet 2 Filed June 9, 1964 CTI United States Patent 3,400,204 METHOD OF MELTING AND SUPPLYING GLASS ALONG A FEEDER DUCT Philip Anthony Maunsell Geil, Kinver, near Stourbridge,

England, assignor to Element Limited, Bilston, England, 2 British company Fiied June 9, 1964, Ser. No. 373,613 Claims priority, application Great Britain, Feb. 29, 1964,

8,595/ 64 10 Ciaims. (Cl. 13-6) ABSTRACT OF THE DISCLOSURE The temperature of a stream of glass flowing from a furnace through a long duct to a feeding chamber is made uniform over the cross-section of the stream by electric current flowing between U-shaped electrode as semblies spaced lengthwise of the duct near the bottom wall of the sarne and by cooling gas directed against the stream surface near the furnace. The current in two longitudinally consecutive sections of the duct is independently controlled by automatic devices in response to temperature and current sensing elements.

This invention reiates to a method of controlling the temperature of molten glass (which term is to be deemed to include vitreous materials generally) while the glass is flowing in a generally horizontal direction along a duct from a furnace charnber containing a body of glass in a molten state to a eeder charnber trom which it is re quired to be delivered for fabrication or other use. 'l'he invention aiso relates to a furnace assembly for carrying out the method.

In the specification of United States Letters Patent No. 3,030434 granted to me, I have described and claimed a method of conducting molten glass along a feeder duct which is long relatively to its cross-sectional dimensions from a furnace charnber to a feeder charnber, such method comprising the steps of flowing the molten glass through said duct, passing an alternating current through said flowing molten glass lengthwise of the direction of flow thereof by communicating said current to said glass at a plurality of positions spaced apart longitudinally of said duct over an area extending for substantially the whoie width and depth of the glass in the duct, and controlling the value of said -current to effect heating of the glass uniformly -between said positions, Said specification also describes and claims a furnace assembly forenabling the method to be carried out.

While this method as described and claimed in thespecification of my prior patent does enabie better control to be exercised than heretofore over the temperature of the glass as it flows along the feeder duct, I have been able to attain a stil1 better degree of controI-by certain improvements in the method as hereinafter descrbed. The

improvements are based upon the realisation that one of the residual causes of temperature diffrence still ccurring in the glass at the exit end of the feeder duct is a top to bottom temperature gradient existing in the glass When this flows into the entrance end of the feeder duct.A further cause is differential heat 1oss at positions around a perimeter of the transverse cross section of the glass in the feeder, such that greater heat loss tends t0 occur through the bottom wall and through the lower portions of the lateral wa1is of the feeder duct than is the case in respect of the upper portions of the laterai walls. Yet another cause is varation in the rate of throughput of the glass, that is to say the quantity of glass flowing in unit time from the exit end of the feeder duct.

Thus trom one aspect the present invention resides in 3,4 0,204 Patented Sept. 3, 1968 ice a method of controlling the temperature of molten glass while flowing along a duct from a furnace chamber to a feeder charnber comprising the steps of; causing said body to flow in an elongated stream having a -free top surface, a bottom surface, and side surfaces connecting said top and bottom surfaces; directing a stream of a gas against the top surface of a longitudinal portion of said stream, the temperature of said gas beng different from the temperature of said glass at said top surface, and passing electr ic current through this longitudinal portion in a path spaced from the top surface of the glass stream and ad jacent the bottom surface. When the top surface temperature of the glass requires to be raised the gas would normaily be constituted by the gaseous products of combustion from burners disposed above the glass.

The invention is also concerned with imp-rovements in a furnace assembly comprising a furnace chamber for containing a body of molten glass, a feeder chamber from which such glass is delivered for use, and an elongated feeder duct extending substantially horizontally =between and conriected to said chambers and having longitudinal side walls and a bottom wall defining a channel for flow of molten glass trom said furnace charnber to said feeder chamber.

The invention provides a plurality of electrode means spaced apart longitudinally of said feeder duct, each of said electrode means comprising a bottom electrode member adjacent the bottom wall and bordering the lowermost portion of the channel, the electrode means beng arranged closely adjacent the walls of the duct to leave the flow channel substantialiy unobstructed, a supply circuit connected to said electrode means for passing electric current longitudinally of said channel between said bottom electrode members of said electrode means through a portion of said glass in said channel adjacent the bottom wall, and heat transfer means disp0sed in said duct at a leve1 above said glass therein, for extracting heat therefrorn along at least part of said duct.

Additional features of this invention will become ap parent from the following detailed description of a preferred embodiment vvhen considered with reference to the accompanying drawings wherein:

FIGURE 1 is a view in vertical cross section along the iongitudinal centre line of a furnace assembly in accordance with the invention;

FIGURE 2 is a graphical representation of typical ternperatures en-countered in the upper and lower layers of glass along the 1ength of the feeder duct;

FIGURE 3 is a partial transverse cross section through the feeder duct showing the electrodes which serve to supply heat to the lower 1ayer and to the lower portions of lateral boundary layers of glass in the duct;

FIGURE 4 is a diagrammatic representation illustrating correlation between the electrode configuration and dimensions and the regions over which lateral and lower 1ayers of glass tend to become cooled through heat loss through the walls of the feeder duct;

FIGURE 5 is a fragmentary view of the arrangement "shown in FIGURE 1 modified to incorporate automatic control of operation of the heat exchange means and of the current flowing between the electrode means.

Referring firstly to FIGURE 1 the furnace assembiy therein shown comprises a glass melting furnace proper 10 for supplying refined glass to a feeder duct 11 for receiving, at its entrance end this supply of refined glass, and a feeder c-hamber 12 through which the glass delivered from the feeder duct is fed out for fabrication or other use.

I have shown by way of exarnple as the glass melting and refining furnace proper a furnace of the form described and claimed in the specification of United States Letters Patent No. 2,899476. Such furnace comprises a furnace charnber 13 of generally rectangular form in plan containing a body 14 of molten glass through which an alternating electric current is passed between sets of electrodes 15 disposed at opposite ends of the furnace chamber, whilst glass batch materials in powdered or divided form are charged onto the surface of the molton body 14 from a batch leader 13a t form a batch crust 16 covering the surface.

In the bottom wal] of the furnace chamber 13 is formed an outlet opening 17 which communicates by way of a duct 18 with a further charnber 19 which operates as the refining chamber.

In the refining chamber 19 the glass is further heate by passage of alternating current between opposed sets of electrodes 20, and the molten body of glass 21 contained in the refining charnber is delivered between the Upper face of a lip or weir 22 and a transverse barrier 23 at the delivery end of the refining chamber.

It will be understood that whilst the rnelting and refining furnace may be of the form illustrated, this particular form of meltng and refining chamber is in fact less prone to produce a top and bottom temperature gradient in the glass emerging from the delivery end of the refining chamber than is the case with melting and refining furnaces wherein heat is supplied to the molten body of the glass exclusiveiy by fuel fed burners disposed in the spaces above the glass in the melting chamber and in the refining chamber where such is separately provided. This will be evident from the fact that where such fuel fed burners are provided heat is conveyed to the glass partly by radiation from the walls of the charnber above the surface of the glass and partly by conduction from the heated air or gas in the space Within the chamber above the surface of the glass and partly in some cases by direct contact betweenthe flame of the burner and the surface of the glass itseif.

In these cases there is thus inherently a rnuch greater tendency to set up a temperature gradient trom top to bottom of the glass with the highest temperature in the upper or surface layer and the lowest temperature in the bottom or lower layer than is the case with the melting and refining furnace illustrated, although such effect is present to some degree even in the letter case.

Whatever form of melting and refining furnace employed may be it is desired that in the feeding chamber 12 the temperature of the glass shall be as uniform as possi-ble at all positions throughout the body of glass 24 contained therein. It is well known that temperature uniormity is important for a number of reasons. While by way of example I have illustrated a feeding chamber wherein discrete gobs of glass such as 25 are caused or permitted to be delivered from an orifice 26 in the bottom wall by means of a reciprocating plunger 27, it will be understood that in ether forrns of feeder furnace glass could be delivered as a continuous stream or sheet and that in such cases it is equaliy important to preserve tem perature uniformity to minimise internal stresses in the formed article or product, and in some cases to minimise non-uniformity of other proporties such as optical properties.

For this purpose the melting and refining furnace is connected to the feeder furnace by the relatively long eeder duct 11 which may have a length of as much as 20 feet which however may vary to snit particular requirements.

The internal width of the duct may be in the range 12 to 20 inches, a typical value being 16 inches. Its height measured internally from the bottom to the upper ends of the side walls as seen in FIGURE 3 may be in the range of 6 to 12 inches, a typical value being 9 inches.

Ths relatively long feeder duct defining a channel of which the cross sectional dimensions are small relative to the Iength afiords the opportunity for correcting variations in the temperature throughout the transverse cross section of glass as this fiows trom one end of the duct to the other. However, the temperature of the glass which would exist at the exit end 28 would vary considerabiy as a functon of the throughput, which typically may vary from a flow corresponding to 10 tons per day to 50 tons per day, it being home in mind that the decrease in temperature from the entrance end 29 and the exit end 28 has heretofore been wholly brought about by natural cooling consisting of heat loss through the walls of the feeder duct itself.

Moreover, as will be evident from the foregoing descr-iption the temperature of the glass at the entrance end 29 is prone to be considerably higher in the upper or surface layer, for example typically 1400 C. in comparison with that which exists in the bottom or lower 1ayer, typically 1100 C. so that each of these two layers and the intervening body of glass which exhibts a corresponding temperature gradient from top to bottom must be brought to a predetermined temperature, for example 1200 C. before the glass reaches the exit end 28 and notwithstanding that the whole furnace assembly may be required to be used at difierent rates of throughput at different times.

Accordngly the feeder duct which comprises a bottom wall 30, side walls such as 31, and a top wa1l 32 entirely enclosing the body 33 of rnolten glass flowing therethrough, the surface 34 of which however is spaced below the top wall 32, is equipped aiong a first or active zone 35-with a heat transfer means indicated generally at 36 for selectively extracting heat from or supplying heat to the upper Iayer of glass. In the active zone 35 the lower layer of glass and the lower portions of the iateral boundary layers are supplied with heat by passing alternating electric current longitudinally through the glass between longitudinaliy spaced electrodes 37 and 38. Althoughtwo only of these are shown at opposite ends of the active zone it will be understood that if desired there may be any number of such longitudinally spaced electrodes provided throughout the active zone.

The heat transfer means 36 may comprise cooling devices 39 such as nozzles for introduction of cooling ar or gas in a direction such that it impinges on the surface 34 of the glass and fuel fed burners 40 which may be alternatively brought into operation in order to suppiy heat to the surface layer of the glass.

The cooling device would normally be brought into operation When the rate of throughput is high, for example at around tons per day. For the lower rates of throughput the burners 40 would be operated.

In the lower layer of glass and throughout the lower portions of lateral boundary layers the temperature of the glass entering the feeder duct at its entrance end will normally be be1ow the predetermined value of temperature required at the exit end, and consequently heat is supplied to these layers internally of the glass by the passage of alternating electric current as aforesaid.

FIGURE 2 illustrates typical temperature curves of which A shows a variation in temperature of the upper layer of glass throughout the length of the active zone, and B shows like variation in the temperature of the glass in the lower Iayer and in the lower portions of the lateral boundary layers.

It will be evident that at the position C, constituting the termination of the active zone, the upper Iayer and the lower Iayer and lower portions of the Iateral boundary layers have temperatures approximating to the predetermined temperature required.

Preferably the active zone 35 occupies about half the total length of the feeder duct. The remainng length constitutes a second or passive zone 41 in which both the upper layer and the lower layer, together with the lateral boundary layers, are caused to approximate still further towards, and preferably attain, the predetennined ternperature at the termination of the passive zone coincident with the exit end 28 of the feeder duct.

For this purpose some heat continues to be supplied to the lower layer and to the lower portions of the lateral boundary layers by passing alternating electric current longitudinally through the glass between electrode 38 and electrode 42.

A certain proportion of this heat is transmitted to the upper layer by conduction through the body of molten glass contained in the passive zone so as partly to compensate for natural cooling in the course of flow of the glass through this zone.

It will be evident, however, that the rate of transmission of heat to and from the glass in the passive zone is at an appreciably lower value than that obtaining in the active zone. Because of the lower rate of heat transference the temperature over a transverse cross section of the glass exhibits a much smaller gradient whch may be virtually zero at the exit end of the feeder duet. There is also little or no tendency for the glass to flow in directions transversely of the desired longitudinal flow path due to convection effects.

To enable the requisite heat to be supplied to the lower layer of glass by means of the electrodes in both the active zone and the passive zone by electric currents of different strength, and to enable heating or cooling of the glass to be effected as required with respect to its upper layer in the active zone, temperature sensing means are provided for sensing the temperatures in the lower and upper layers of glass at varous positions along the length of the feeder duet. Thus, for example thermo-couples TC1, TC3 and TC5 are immersed in the upper layer of glass at the ends of the feeder duet and at a position approximately midway along its length corresponding to the juncton between the active and passive zones. Similarly thermo-couples TC2, TC4 and TC6 enable the temperature of the lower layer of glass to be sensed at corresponding positions.

According to the indication furnished on each of the lower thermo-couples TC2, TC4, TC6 the voltage between electrodes 37 and 38 on the one hand and electrodes 38 and 42 on the other hand, may be adjusted for example by provision of adjustable voltage transtormers T1 and T2 connected as shown in FIGURE 1. The transformer T1 and the electrodes 37 and 38 constitute one branch circuit and the transformer T2 and the electrodes 38 and 42 constitute another branch circuit.

The primary windings of these transformers have terminals 11 and t2 respectively whch may be connected to respective phascs of a poly-phase supply or, if desired, in series or in parallel to a single phase supply.

In FIGURE 5 whch shows a modified arrangement, parts corresponding to those already described in relation to FIGURE 1 are designated by like characters of reference with the prefix 100 so as to lie in the series 100 to 200.

Referring firstly to the supply circuit for the electrode means respective branches thereof: incorporating variable voltage transformers T103 and T104 feed the electrodes 137 and 138 on the one hand and the electrodes 138 and 142 on the other hand.

The current passing between each of these pairs of electrodes is stablished by the provision of a current stabilising means comprising, for exarnple, current transformers CT1 and CT2 providing an input to computers or current controlling devices 150 and 151 for the two portions of the supply circuit.

An operative coupling whch may be mechanical, electromagnetic, or of other suitable form as indicated at 152 and 153 serves to connect the computers or current controlling devices with the respectve tarnsformers T103 and T104.

The computers or current controlling devices 150 and 151 also receive an input trom respective therrno-couples TC104 and TC106 which are disposed at a position such as to sense the temperature of the glass flowing out of each of the controlled zones. The electrodes 137 and 138 with their associated transformer T103 constitute one branch circuit whereas the electrodes 138 and 142 and their associated transformer T104 constitute a furthei branch circuit.

The input from the current transformers CT1 and ST2 tends to set the voltage of the associated tarnsformer T103 and T104 as the case may be to a value such as to maintain constant current through the lower layer of glass.

Consequently this feature of the supply circuit inherent ly compensates to some extent for variations in the temperature of glass flowing into the feeder duet and whch arise from variations in the temperature of the glass flowing into it. If the glass entering the feeder duet is above the average temperature encountered over the period of operation its electrical resistance is lower, but since the current flowing through it is stablished to at least approximately constant or datum value the electrical power dissipated in the glass will be reduced with consequent temperature whch would otherwise occur at the exit end of the feeder duet.

The supply circuit however also provides for pre-setting of the constant or datum value of current by means of a signal 0r input supplied to each of the computers or controllers or 151 from their respective thermo-couples TC104, TC106 so as still further to minimise any departure from the desired temperature of glass flowing out from the exit end of the feeder duet.

It will be apparent that by applying such control in both the active and passive zones very precise control over the temperature of the glass leaving the feeder duet of the exit end is possible. Variations in the tempemture of the glass flowing into the passive zone are substan tially reduced by virtue of the operaton of the computer or current controller 150 in the supply circuit for the active zone, so that the current controller or computer 151 is subjected to error signals by way of its thermocouple 106 and current transformer CT2 of a very much lower order.

The heat transformer means comprising the cooling devices 139 and fuel fed burners 140 may also advantageously be controlled automatically by means of a setting device 154 which is connected mechanically, electromagnetically, or by other suitable means as indicated at 155 and 146 to the group of cooling devices 139 and burners 140.

The setting device is itself controlled by signals received from thermo-couples TC101 and TC103 and may be arranged to bring the cooling devices or burners selectively into operation and to adjust these to provide the required degree of heating or cooling to achieve a predetermined temperature at the thermo-couple TC103.

Referring now specifically to the construction of electrodes employed as illustrated in FIGURE 3 each such electrode is U-shaped and comprises a bottom electrode member 43 in the orm of an elongated plate or strip of molybdenum or other suitable metal whch does not react with or discolour the glass and is not destructively eroded thereby. At its ends are upstanding lateral electrode members 44 fixed to the bottom electrode member in any suitable manner, for example by being bolted thereto or welded thereto, and also in the form of elongated plates or strips. Each of the electrode members 43 and 44 are disposed in a vertical plane at right angles to the centre line of the feeder duct.

In order to enable each electrode to be connected to the supply circuit and also to provide support for the electrode the bottom electrode member 43 has fixed to it at laterally projecting lead-in portion 45 whch extends through an opening 46 in the adjacent lateral wall 31 of the feeder duet at the lower end thereof.

The electrode is partly supported by contact between the lower edge of the member 43 with the bottom wall of the duet and partly by the cantilever support furnished by the lead-in portion 45.

The lead-in portion 45 forms part of a structure disposed within the opening 46 of a side wall 31. The leadin porton 45 which isin the form of a stem of circular cross section is tightly embraced by a sleeve 50 of copper, the inner end portion of which projects nto but not through the opening 46 and engages with the walls thereof to support the leadin portion 45 at its rearward end.

The lead-in portion or stem 45 is forrned of molybdenum which although capable of withstanding immer sion in the rnolten giass oxidises rapidly in air at temperatures in excess of 500 C. The copper sieeve 50 is cooled at itsouter end by means of air delivered through a duct 49 and directed on to fins 51 formed at the outer end of the sleeve externally thereof and disposed in an enlargement 52 at the outer end of the opening 46.

Within the opening 46 the inner part of the lead-in portion or stem 45 is protected by means of a sealing collar 53 which also serves to locate and support the stem at its forward end within the opening. Further protection is given by a filling of a porous protective substance such as partcles of nickel aluminide disposed in the space 54 while ths part of the stem may be further protected by a coating of a heat-resistant enamel having a high borosilicate content. Inert gas such as nitrogen or hydrogen may also be fed in through a transverse passageway 55 to flow axially inwardly through a space between the opening 46 and the sleeve 50 so as to provide an inert or non-oxidsing atmosphere within the space 54.

Electrical connection to the outer end of the stem is by means of a cable 47, cables for all of the electrodes being contained in a duct 48.

FIGURE 4 represents dagrammatically the relationship between the configuration of each of the electrodes 37, 38, 42 as viewed in a direction longitudinally of the feeder duct and the regions of the peripheral margin of the transverse cross section of the glass trom which heat loss is most pronounced. For the sake of simplicity only the electrode members 43, 44 are shown, the lead-in portion or stem 45 and assocated structure being omitted.

The dashed lne 56 represents the inner boundary of Iayers of glass which tend to become cooled by heat loss through the walls of the feeder duct to an extent which is rather greater than that which occurs in respect of the upper portions of the lateral boundary layers (that is above the line 56 at each of its ends).

The height of each of the electrode members 44 is thus selected so that these at least approximately coincide with the lower portions of the 1ateral boundary layers trom which heat loss is greatest. The width of the electrode members 44 is also selected with ths criterion in view as is also the height of the lower electrode member 43 so that as a whole there is approximate coincidence between the part of the cross sectional area bounded by the line 56 and by the bottom and lateral walls of the feeder duct and the area presented longitudinally of the duct by the electrode as a whole. Typically the electrode members 44 have -a height such that their upper ends lie approximately midway between the bottom of the duct and the surface of the glass. It will be evident that the height of the lower electrode mernber 43 (measured norrnal to the bottom wa1l of the duct) is only a relatively small proportion of the depth of the glass in the feeder duct, for example of the order of 20%, and similarly the width of each of the electrode members (measured normal to the adjacent side wal!) is of the order of 12% of the overall width of the feeder duct.

Consequently the presence of each of the electrodes creates very little impediment or obstruction to longitudinal flow of glass along the duct.

The level of the glass in the duct maintained constant by level sensing means 200 in the chamber 19, such means 200 being connected to the batch loader 13a, which is controlled automatically or manually in dependence on the signal received from the sensing means 200.

In some cases where there is lttle or no heat loss from the side or bottom walls the electrode member 44 alone may be provided in the 1atter case these being carried at the lower ends of dependent support elements.

What I claim then is:

1. In a furnace assembly including a furnace chamber for contaning a body of molten glass and a eeder chamber communicating with said furnace chamber for delvering such glass for use, the improvement which comprises:

(a) a horizontally elongated feeder duct connecting said chambers -and defining a channe1 for flow of rnolten glass trom said urnace charnber to said feeder charnber,

(1) said duct having a bottom wal] and two longitudinal side walls;

(b) a plurality of electrode means spaced longitudinally from each other in a perpheral zone of said duct adjacent said walls so as to leave said channel substantiaily unobstructed,

(2) each of said electrode means ncluding a bottom electrode member adjacent said bottom wall and bordering the lowerrnost portion of said channel;

(c) supply circuit means connected to said electrode means for passing electric current through said bottom electrode members of said electrode means and longitudinally through a portion of said glass in said channel adjacent said bottom wa1l; and

(d) heat transfer means in said duct for extracting heat from a portion of said glass in said duct upwardly spaced from said bottom wal].

2. In an assembly as set forth in claim 1, said duct having two longitudinal portions respectively defining a first zone of said channel adjacent said furnace charnber and a second zone of said channel adjacent said feeder chamber, respective pairs of said electrode means being arranged in said duct at the longitudinal ends of said zones, said supply circuit means including two branch circuuit means respectively connected to said pairs of electrode means for passing currents of different strength through said glass in said zones respectiveiy, said heat transfer means being effective for extracting heat frorn said portion of said glass in said first zone.

3. In an assembly as set forth in claim 1, means for maintaining said glass in said channel at a predetermined level, each electrode means including a substantially U- shaped assembly of electrode rnembers including said bottom electrode member and two lateral electrode members respectively adjacent said side wails, said electrode assembly being downwardly spaced trom said level and extending in a plane transverse of the direction of elongation of said duct; said side walls, said level, and said heat exchanging means defining therebetween a space above said level and said heat exchanging means including cooling means for delivering a cooling fluid to said space, heating means for heating said space, -and control means for selectively operatng said cooling means and said cooling means.

4. In an assembly as set forth in claim 1, one of said side walls being formed with a plurality of apertures respectively adjacent said electrode assemblies, each elec trode assembly further ncluding lead-in means extending through the associated aperture, and supporting means in the aperture supporting said lead-in means, said electrode assembly being cantilevered on said lead-in means and engaging a portion of one of said walls spaced from said aperture.

5. In an assembly as set forth in claim 1, said supply circuit means including current controlling means -for controlling the strength of the current passing through said electrode means, temperature sensing means responsive to the temperature of said porton of said glass adjacent said bottom wal1 and operatively connected to said current controllng means forchanging said strength in response to the sensed temperature in a sense tending to maintain constancy of temperature, and current stabilizing means responsive to said strength and operatively connected to said current controlling means for changing said strength also in a sense tending to maintain constancy of temperaturc.

6. In an assembly as set forth in claim 1, heating means for heating said porton of said glass upwardly spaced from said bottom wa1l in said duet, and control means for selcctvcly operating said heat transfer means and said heating means.

7. A method of controlling the temperature of a body of molten glass which comprises:

(a) causing said body to flow in an elongated stream having a free top surfacc and a bottom surface;

(b) directing a stream of gas against said top surface in a longitudinal portion of said stream,

(1) the temperature of said gas beng diierent from the temperature of said body of glass at said top surface; and

(c) passing electric current through said longitudinal portion in a path spaced from said top surfacc and adjacent said bottom surface.

8. A method as set forth in claim 7, wherein said stream has side surfaces connecting said top and bottom surface, and said path is substantially U-shaped in cross section and having respective parts adjacent said bottom surface and said side surfaces.

9. A method as set forth in claim 8, which further comprises passing another electric current through another longtudinal portion of said stream in another path spaced from said top surface and adjacent said bottom surface, said electrc currents being passed through said longitudinal portions simultancously and being of different strength.

10. In a furnace assembly includng a furnacc cham ber f0r containng a body of molten glass and a feeder chamber communicating with said furnacc chan1ber for delivering such glass for usc, the improvement which comprises:

(a) a horizontally elongated feeder duct connecting said chambers and defining a channcl for flow of rnolten glass from said furnace chamber t0 said fecder chamber,

(I) said duet having a bottom wall and two longitudinal sde walls;

(b) a plurality of electrode means spaced longitudi nally from each other in a peripheral zone of said duet adjacent said walls so as to leave said channel substantaily unobstructed;

(c) supply circuit means connected to said elcctrode means f0r passing electric current through said electrode means and longitudinally through said glass in said channel, said supply circuit including (1) current controlling mcans,

(2) tempcrature sensing mcans arranged for therrnal Contact with the flowing glass outside said furnacc chamber,

(3) means operativcly connecting said temperature sensing means and said current controlling means for operating said currcnt controlling rneans in response to the sensed tempcrature,

(4) current sensing mcans for sensing the current passing through said glass, and

(5) means operatively connecting said current sensing means to said current controlling mcans for stabilzing said current.

References Cited UNITED STATES PATENTS 2,512,761 6/1950 Arbeit 13-6 2866,838 12/1958 Paxton 13-6 3182112 5/1965 Torok 13-6 BERNARD A. GILHEANY, Primary Emmlmr.

H. B. GILSON, A ssstant Examiner. 

